1. Field of the Invention
The present invention relates to a controller for a magnetic base drill unit, and particularly to a controller for a magnet base drill unit wherein the feeding of a drill unit (an electric drill) is made by a feed motor, and the drill unit is restored to its initial position by causing the drill unit to rise at a high speed after the termination or completion of the outting by the drill.
2. Description of the Prior Art
Various kinds of magnetic base drill units have already been proposed, which comprise a drill unit provided with an annular hole cutter, a magnetic base for causing the drill unit to be magnetically adhered onto a workpiece for fixing, a feed motor for automatically feeding the drill unit to the workpiece, and the like. A magnetic base drill unit has also been proposed, wherein the drill unit is restored to its initial position after the hole cutting work by the drill unit is completed.
For instance, in the specification of U.S. Pat. No. 4,789,274 issued on Dec. 6, 1988 and assigned to the same assignee as the present invention, a technique is described for enhancing the efficiency of the hole cutting work by making the restoring (rise) speed of the drill unit after the termination of the hole cutting work faster than the feed (fall) speed of the drill unit during the hole cutting work. Specifically, in the magnetic base drill unit described in the above U.S. patent, rotation of the feed motor is transmitted to the drill unit through a reduction gear during the hole cutting work, while it is transmitted to the drill unit without through the reduction gear after the termination of the hole cutting work.
However, such magnetic base drill unit requires a mechanism for disconnecting the reduction gear connected to the feed motor, so the construction of the magnetic base drill unit becomes complicated and large-sized. In order to dissolve such problem, it is considered that a d.c. motor is utilized as the feed motor, the polarity of the power supply to the motor is made invertible, and its voltage or current value is made variable. That is, by making the voltage applied to the feed motor lower during the hole cutting of the drill unit, while by making it higher after the termination of the hole cutting, the rise speed of the drill unit can be made faster than the fall speed thereof.
The above described technique had the following problem The fall and rise speeds of the drill unit can be altered, as previously stated, however, if the voltage and/or current applied to the feed motor becomes excessively large during the hole cutting (falling of the drill) because of an unexpected factor, the fall speed of the drill unit becomes excessively large, and the annular cutter attached to the arbor assembly of the drill unit may be broken or the drill motor may be burn out.